My father رحمه الله said, it was narrated to me by ʿAbd Allāh bin al-Ḥassan al-Mu’adab, from Aḥmed bin ʿAlī al-Aṣbahāni, from Ibrāhīm bin Moḥammed al-Thaqafi, from Moḥammed bin Yaḥya, from Moḥammed bin ‘Isḥāq, from his father Hārūn al-ʿAbdi, from Abī Saʿīd al-Khidri, who said:

One day, The Messenger of Allāh صلى الله عليه وآله was sitting with a group of his companions, among them ʿAlī bin Abī Ṭālib عليه السلام, when he (the Prophet) said: “Whoever says “There is no god but Allāh” enters Paradise.”
Two men from his companions said: “We say “There is no god but Allāh!”
The Messenger of Allāh صلى الله عليه وآله said: “The testimony of “There is no god but Allāh!” is accepted only from him (ʿAlī ) and his Shiʿa, the ones whom our Lord has taken a covenant with.”
The two men said: “Then, we [also] say “There is no god but Allāh!”So the The Messenger of Allāh صلى الله عليه وآله placed his hand upon the head of ʿAlī عليه السلام and then said: “The sign of that (i.e. one who is a Shiʿa of ʿAlī, and his testimony is accepted), is he does not break his oaths given to him (ʿAlī ), does not sit in the place of his sitting, and does not reject his words.”

أبي رحمه الله قال حدثني عبد الله بن الحسن المؤدب عن أحمد بن علي الأصبهاني عن إبراهيم بن محمد الثقفي عن محمد بن يحيى عن محمد بن إسحاق عن أبي هارون العبدي عن أبي سعيد الخدري قال كان رسول الله صلى الله عليه وآله ذات يوم جالسا وعنده نفر من أصحابه فيهم علي بن أبي طالب عليه السلام إذ قال من قال لا إله إلا الله دخل الجنة فقال رجلان من أصحابه فنحن نقول لا إله إلا الله فقال رسول الله صلى الله عليه وآله إنما تقبل شهادة ان لا إله إلا الله من هذا ومن شيعته الذين أخذ ربنا ميثاقهم فقال الرجلان فنحن نقول لا إله إلا الله فوضع رسول الله صلى الله عليه وآله يده على رأس علي عليه السلام ثم قال: علامة ذلك أن لا تحلا عقده ولا تجلسا مجلسه ولا تكذبا حديثه