بُلُوغُ الْمَرَامِ
Bulugh al-Maram 436
The Book of Prayer
وَلَهُ عَنْ عِمْرَانَ بْنِ حُصَيْنٍ: { ثَمَانِيَ عَشْرَةَ } [1] .
The narration of Abu Dawud from 'Imran bin Husain (RA) has: "eighteen days."
Sunnah.com reference: Book 2, Hadith 348