Sahih Al Bukhari (405)

عن أنس: أن النبي صلى الله عليه وسلم رأى نخامة في القبلة، فشق ذلك عليه، حتى رئي في وجهه، فقام فحكه بيده، فقال: (إن أحدكم إذا قام في صلاته، فإنه يناجي ربه، أو، إن ربه بينه وبين القبلة، فلا يبزقن أحدكم قبل قبلته، ولكن عن يساره أو تحت قدميه). ثم أخذ طرف ردائه، فبصق فيه، ثم رد بعضه على بعض، فقال: (أو يفعل هكذا).

Anas ibn Malik (may Allah be pleased with him) narrated: The Prophet (peace be upon him) saw some sputum in the direction of the Qiblah (on the wall of the mosque) and he disliked that and the sign of disgust was apparent from his face. So, he got up and scraped it off with his hand and said, “Whenever anyone of you stands for the prayer, he is speaking in private to his Lord or his Lord is between him and his Qiblah. So, none of you should spit in the direction of the Qiblah but one can spit to the left or under his foot.” The Prophet (peace be upon him) then took the corner of his sheet and spat in it and folded it and said, “Or you can do like this.”