I believe he mentioned that the Prophet ﷺ said, "People must stop raising their eyes toward the sky during prayer, or else Allah will snatch away their sight."
حدثنا خلف قال حدثنا المبارك قال حدثنا الحسن
وأراه ذكر النبي ﷺ قال لينتهين أقوام يرفعون أبصارهم في الصلاة إلى السماء أو ليخطفن الله أبصارهم
Its isnad is sahih. It has occurred previously at and will come again at
. It is found in Muslim 1/321, no. 429, in the Book of Prayer, chapter on the prohibition of raising the eyes to the sky, with wording close to this, and something similar to it was narrated by Bukhari 1/191, same book and chapter, with the wording, "What is the matter with people who raise their eyes," and by Ibn Majah 1045, and Tabarani in Al-Kabir 2/220, no. 1817.