“Ibn Hajar Asqalani” quotes from “Tabari” who has quoted from “Ibn Abbas” with authentic document:
“قال: لما نزلت هذه الآیه وضع رسول الله یده علی صدره و قال: انا المنذر . واوما الی علی و قال انت الهادی بک یهتدی المهتدون بعدی”
When this verse was sent down:
وَيَقُولُ الَّذِينَ كَفَرُوا لَوْلَا أُنْزِلَ عَلَيْهِ آيَةٌ مِنْ رَبِّهِ إِنَّمَا أَنْتَ مُنْذِرٌ وَلِكُلِّ قَوْمٍ هَادٍ.
“The unbelievers say: 'Why has no sign been sent down to him from his Lord? You are only a warner, and there is a guide for every nation.
Sura Al-Rad- verse 7
Prophet Muhammad [PBUH] put his hand on his chest and said: I’m the warner and the he mentioned to hadrat “Ali bin abi Talib” and said: after me, guide seekers are guided by you.
Fath al-Bari – vol. 10, p 260