Imam Abu Dawud (may Allah have mercy on him) says: I heard Imam Ahmad bin Hanbal (may Allah have mercy on him) describe the line; he said: "Like this," meaning widthwise, in the curved shape of the crescent moon.
Imam Abu Dawud (may Allah have mercy on him) says: I heard Musaddad, who related on the authority of Ibn Dawud that the line should be drawn lengthwise.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3469