Sahih Al Bukhari (740)

عن أبي هريرة وأبي سعيد قالا: أن رسول الله صلى الله عليه وسلم رأى نخامة في جدار المسجد، فتناول حصاة فحكها، فقال: (إذا تنخم أحدكم، فلا يتنخمن قبل وجهه، ولا عن يمينه، وليبصق عن يساره، أو تحت قدمه اليسرى).

Abu Hurayrah and Abu Sa`id (may Allah be pleased with them) narrated that Allah’s Messenger (peace be upon him) saw some expectoration on the wall of the mosque; he took gravel and scraped it off and said, “If anyone of you wanted to spit he should neither spit in front of him nor on his right but he could spit either on his left or under his left foot.”