Sunan Abi Dawud Book 2, Hadith 299
The Prophet (ﷺ) said: When one of you prays, he should put something in front of his face, and if he can find nothing , he should set up his staff; but if he has no staff, he should draw a line; then what passes in front of him will not harm him.
The Prophet (ﷺ) led them in prayer at al-Batha', with a staff set up in front of him. (He prayed) two rak'ahs of the Zuhr prayer and two rak'ahs of the 'Asr prayer. The women and the donkeys would pass in front of the staff.
The Prophet (ﷺ) said: ...... He then narrated the tradition about drawing the line.
Sufyan said: We did not find anything by which we could reinforce this tradition, and this has been narrated only through this chain.
He ('Ali b. al-Madini, a narrator) said: I said to Sufyan: There is a difference of opinion of the name (Abu Muhammad b. 'Amr). He pondered for a moment and then said: I do not remember except Abu Muhammad b. 'Amr Sufyan said: A man had come to Kufah after the death of Isma'il b. Umayyah ; he was seeking Abu Muhammad until he found him. He asked him (about this tradition) but he became confused. Abu Dawud said: I heard Ahmad b. Hanbal who was questioned many times how the line should be drawn. He replied: In this way. horizontally like crescent.
Abu Dawud said: I heard Musaddad say: Ibn Dawud said: The line should be drawn perpendicularly.
Abu Dawud said: I heard Ahmad b. Hanbal describing many times how the line should be drawn. He said: In this way horizontally in the round semi-circular form like the crescent, that is (the line should be) a curve.