What is the evidence for the permissibility of clearing the throat out of necessity, even if two letters are produced?

Question : Now, if someone asks, what is the evidence for the permissibility of clearing the throat out of necessity, even if two letters are produced?

Answer : The evidence is the hadith of 'Ali (may Allah be pleased with him) that he had two entrances through which he would enter upon the Prophet (peace and blessings be upon him). So when he entered upon him while he was praying, he would clear his throat to him as a sign that he was occupied with his prayer.