It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both): "He (on completing the prayer) said the salam only once, while still facing towards the qiblah."
Ibn Sa'd transmitted this report in his book al-Tabaqat al-Kubra. The narrator in its chain, Muhammad bin Umar al-Waqidi, is da'if (weak) according to the majority of hadith scholars; however, the practice of saying the salam to one side in prayer is established by other authentic reports and supporting evidence.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / صفة الصلاة / 527
Hadith Gradingمحدثین:إسناده صحيح
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :صحيح ، عمدة القاري شرح صحيح البخاري: (8 / 18) » «انفرد به المصنف من هذا الطريق»