Revelation would come down upon the Messenger of Allah ﷺ while he was on his mount, and it would press its chest to the ground under the weight.
قال حدثنا سليمان بن داود قال أخبرنا عبد الرحمن عن هشام بن عروة عن أبيه
إن كان ليوحى إلى رسول الله ﷺ وهو على راحلته فتضرب بجرانها
Its isnad is sahih. Haythami said in 8/257, "Its narrators are the narrators of the Sahih." This has already come before.