Hadithcore
Sahih Muslim, 2255 c
sahih

`Amr b. Sharid reported on the authority of his father that Allah's Messenger (ﷺ) asked him to recite poetry, the rest of the hadith is the same, but with this addition:

"He (that is Umayya b. Abu Salt) was about to become a Muslim", and in the hadith transmitted on the authority of Ibn Mahdi (the words are) "He was almost a Muslim in his poetry."

وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا الْمُعْتَمِرُ بْنُ سُلَيْمَانَ، ح وَحَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ مَهْدِيٍّ، كِلاَهُمَا عَنْ عَبْدِ اللَّهِ بْنِ عَبْدِ الرَّحْمَنِ الطَّائِفِيِّ، عَنْ عَمْرِو، بْنِ الشَّرِيدِ عَنْ أَبِيهِ، قَالَ اسْتَنْشَدَنِي رَسُولُ اللَّهِ صلى الله عليه وسلم ‏.‏ بِمِثْلِ حَدِيثِ إِبْرَاهِيمَ بْنِ مَيْسَرَةَ وَزَادَ قَالَ ‏"‏ إِنْ كَادَ لَيُسْلِمُ ‏"‏ ‏.‏ وَفِي حَدِيثِ ابْنِ مَهْدِيٍّ قَالَ ‏"‏ فَلَقَدْ كَادَ يُسْلِمُ فِي شِعْرِهِ ‏"‏ ‏.‏

References2 variants
In-Book Reference
Book 41, Hadith 2
USC-MSA web (English) reference
Book 28, Hadith 5603 (deprecated numbering scheme)