The Prophet peace be upon him said
He who eats seven ‘Ajwa dates every morning will not be affected by poison or magic on the day he eats them.” Sahih Bukhari 7:65:356
Many of the Christian missionaries say, eat ajwa dates and then we will give you poison. The answer to that is
1. We should understand hadith with the understandings of Companions of Prophet peace be upon him and tabiyeen.
2. Not a single Companion understood it like if someone eats Ajwa and then eat poison, it will not effect. Otherwise they would be the first ones to use this hadith against (non Muslims)
3. Prophet Muhammad peace be upon him said
ومَن تحسَّى سمّاً فقتل نفسه فسمُّه في يده يتحساه في نار جهنم خالداً مخلداً فيها أبداً
Whoever takes poison and kills himself, his poison will be in his hand and he will be sipping it in the Fire of Hell for ever and ever. [Narrated by al-Bukhaari, 5442; Muslim, 109.]
It was narrated from Thaabit ibn al-Dahhaak (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever kills himself with something in this world will be punished with it on the Day of Resurrection.” [Narrated by al-Bukhaari, 5700; Muslim, 110.]
Comment: So clearly Eating poison is killing yourself and it is prohibited, the hadith in discussion means if someone eat dates in the morning with strong certainty and full submission, then if some scorpion or any other poisoning animal bite him by any chance then Insha’Allah no poison will effect him but he should not think that dates will save him but it is Allah who saves.
Shaykh Ayman bin Khaled said
It means:
1- you are not allowed to take poison or anything may harm your body.
2- ‘Ajwa dates is one of the means that prevent being poisioned, With The Permission of Allah.
3- being protected against poison by eating these dates is conditional to the will of Allah; if Allah will its effect happen and if He does not Will so then posien will take effect.(end quote) http://www.ahlalhdeeth.com/vbe/showthread.php?t=11617
Note: Suicide will not make a person Non Muslims
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It is mentioned in Saheeh Muslim (113):
“When the Prophet (sallallaahu ‘alayhe wa sallam) made hijrah to Madeenah, Tufayl ibn ‘Amr came as well, along with a man from his tribe. This man became ill when he first reached Madeenah and his illness became so severe that he took a knife and slit his wrist (over the top, or possibly near the knuckles), and the blood spilled out until he died. Tufayl then saw him in a dream, in a good vision, except that his hands were wrapped up.
So he asked him, “What has your Lord done with you?” He replied, “He has forgiven me because of my hijrah to His Prophet (sallallaahu ‘alayhe wa sallam). The he asked, “Why are your hands wrapped up?” He said, “It was said to me: We shall not fix something you have corrupted yourself!” So Tufayl relayed this to the Messenger of Allaah (sallallaahu ‘alayhe wa sallam), so he said: “O Allaah! And forgive his hands (too)!”(end quote)
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Al-Qaadhee ‘Iyadh (d.544) said in Ikmaal al-Mu’lim (1/403):
In this hadeeth is proof for Ahlus-Sunnah for what they say, that Allaah may forgive the sins of whomever He wants, and it explains the hadeeths before it that might seem to give the false impression that someone who commits suicide faces the eternal threat of remaining (in the Fire) forever.(3) It is also a refutation of the Khawaarij and Mu’tazilah.
Shaykh Abdul Aziz Bin Baz said
A person who has killed himself should be washed, prayed over and buried with the Muslims, because he is a sinner but he is not a kaafir. Killing oneself is a sin but it is not kufr. If he killed himself – we seek refuge with Allaah – he should be washed, shrouded and prayed over, but the khaleefah and important people should not pray for him, by way of rebuke, lest anyone think that they approve of what he did. If the khaleefah, ruler, judges, president or mayor do not pray over him in order to denounce this action and announce that it is wrong, then this is good, but some of the Muslims should still offer the (funeral) prayer for him. [Kitaab Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah li Samaahat al-Shaykh ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz (may Allaah have mercy on him) – vol. 13, p. 122]
see also: http://islamqa.com/en/ref/12316
and this: http://www.letmeturnthetables.com/2011/07/hadith-ajwa-dates-and-science.html