Hadith: Say 'there is no god but Allah', and I will testify for you by it on the Day of Judgment

عَنْ ‌أَبِي هُرَيْرَةَ رضي الله عنه قَالَ:
قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ لِعَمِّهِ: «قُلْ: لَا إِلَهَ إِلَّا اللهُ، أَشْهَدُ لَكَ بِهَا يَوْمَ الْقِيَامَةِ»، قَالَ: لَوْلَا أَنْ تُعَيِّرَنِي قُرَيْشٌ، يَقُولُونَ: إِنَّمَا حَمَلَهُ عَلَى ذَلِكَ الْجَزَعُ لَأَقْرَرْتُ بِهَا عَيْنَكَ. فَأَنْزَلَ اللهُ: {إِنَّكَ لا تَهْدِي مَنْ أَحْبَبْتَ وَلَكِنَّ اللهَ يَهْدِي مَنْ يَشَاءُ } [القصص: 56].
[صحيح] - [رواه مسلم] - [صحيح مسلم: 25]

Abu Hurayrah (may Allah be pleased with him) reported:
The Messenger of Allah (may Allah's peace and blessings be upon him) said to his uncle: "Say 'there is no god but Allah', and I will testify for you by it on the Day of Judgment." He said: "Were it not for the fear that the Quraysh may blame me and say that it was the fear (from imminent death) that induced me to do so, I would have certainly delighted your eyes with it." So, Allah revealed: {You cannot guide whoever you like, but Allah guides whom He wills.} [Surat al-Qasas: 56]
Sahih/Authentic. - [Muslim]

Explanation

The Prophet (may Allah’s peace and blessings be upon him) asked his uncle Abu Tālib, while in the throes of death, to utter the testimony of faith, so that he could intercede for him with it on the Day of Judgment and testify that he was a Muslim. But he refused to utter it, fearing that the Quraysh would revile him and say that he embraced Islam out of fear of death and weakness! He said to the Prophet (may Allah’s peace and blessings be upon him): Were it not for that, I would have made you joyful by saying the testimony, fulfilling your wish and making you pleased. So, Allah Almighty revealed a verse indicating that the Prophet (may Allah’s peace and blessings be upon him) cannot guide people in the sense of inspiring them to accept Islam, but it is Allah Almighty alone Who grants such guidance to whomever He wills; and that the Prophet (may Allah's peace and blessings be upon him) guided them by clarifying, showing, and calling them to the straight path.