(Imām Bukhārī narrates): Musa bin Ismail narrated the hadith to us, he said: Abu Awanah narrated the hadith to us, from Asim and Husain, from Ikrimah, from Hazrat Ibn Abbas (Allāh be pleased with them both), he narrates that

the Prophet ﷺ stayed nineteen days shortening (the prayer), so when we travelled, if we stayed nineteen days, we would shorten (offer the prayer of four rak'ahs as two rak'ahs), and if we stayed more than that, we would complete the prayer.