Shortening the salam is a sunnah. Ali bin Hujr said, Abdullah bin Mubarak explained that this means one should not draw it out at length.
حدثنا علي بن حجر أخبرنا عبد الله بن المبارك وهقل بن زياد عن الأوزاعي عن قرة بن عبد الرحمن عن الزهري عن أبي سلمة
حذف السلام سنة قال علي بن حجر قال عبد الله بن المبارك يعني أن لا يمده مدا
No references are available.
Its chain of transmission is daif. Abu Dawud also narrated it, no. 1004, from the hadith of Uzai through this same chain. Ibn Khuzayma graded it sahih, no. 734, as did Hakim according to the criterion of Muslim, 1/231, and Dhahabi agreed with him. Zuhri was a mudallis, and I did not find an explicit statement that he heard it directly.The saying of Ibrahim Nakhai, and I did not find it.