Narrated Ash-Shu`bi:

"`Adi took a white rope (or thread) and a black one, and when some part of the night had passed, he looked at them but he could not distinguish one from the other. The next morning he said, "O Allah's Apostle! I put (a white thread and a black thread) underneath my pillow." The Prophet (ﷺ) said, "Then your pillow is too wide if the white thread (of dawn) and the black thread (of the night) are underneath your pillow! ""

Chapter: "...And eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of the night).. ." (V.2:187)

📖 Sahih al-Bukhari — Sahih al-Bukhari 4509