Abu Wa'il (may Allah have mercy on him) relates: It reached me that Sayyiduna Abu Bakr (may Allah be pleased with him) met Sayyiduna Talhah (may Allah be pleased with him), and he asked: "What is the matter, that I see you so worried?" He replied: "There is a word that I heard from the Messenger of Allah (peace and blessings be upon him); he (peace and blessings be upon him) said that it makes (salvation and Paradise) obligatory, but I was not able to ask him (peace and blessings be upon him) about it (what it is)." At this Sayyiduna Abu Bakr (may Allah be pleased with him) said: "I know well what that word is." He asked: "What is it?" Sayyiduna Abu Bakr (may Allah be pleased with him) said: "That word is 'La ilaha illallah' (There is no god worthy of worship except Allah)."
Hadith Referenceمسند أبي يعلى الموصلي / مسند أبي بكر الصديق رضي الله عنه / 102
Hadith Gradingمحدثین:رجاله ثقات
Hadith Takhrij«حکم الحدیث مسند ابی یعلی محققاً تنسیق حسين سليم أسد - مکتبہ شاملہ :قال حسين سليم أسد: رجاله ثقات . » «قال سعيد بن محمد السناري: ضعيف» «أخرجه المروزى في "مسند أبى بكر" [رقم 12، 13، مسند ابویعلیٰ ترقیم الاثری: 97»