Salatul Layl was practiced by the Prophet (SAWA) and Imam Ali (AS) and Lady Khadija from the beginning of Islam. Sincere believers used to do it as a recommended worship. We read in Quran (Verily, your Lord knows that you stand in praying at night, less than two thirds of the night or half of the night or a third of the night and also a number of believers with you) Sura Al-Muzzammil, verse 20. Sura Al-Muzzammil was from the very early Suras revealed on the Prophet (SAWA) in Makkah.
Wassalam.