´It was narrated that 'Ali said:` "I had two times when I would enter upon the Messenger of Allah (ﷺ), one at night and one during the day. When I entered at night he would clear his throat (to tell me to come in)."
Hadith Referenceسنن نسائي / كتاب السهو / 1213
Hadith Gradingالألبانی:ضعيف الإسناد | زبیر علی زئی:ضعيف، إسناده ضعيف، ابن ماجه (3708) مسند احمد (1/ 80 ح 608) فى سماع عبد الله بن نجي من على رضي اللّٰه عنه نظر. والحديث الآتي (1214 وسنده حسن) يغني عنه. انوار الصحيفه، صفحه نمبر 330
1؎ : That is, he would clear his throat to grant permission to enter, and in some manuscripts, instead of «تنحنح», it is «سبّح», and this is more likely, because in the subsequent narration it is mentioned that clearing the throat was a sign of non-permission. It is also possible that it had two forms: one indicating permission, and the other indicating non-permission. And Allah knows best.
´'Abdullah bin Nujayy narrated that his father said:` "Ali said to me: 'I was so close to the Messenger of Allah (ﷺ), closer than anyone else. I used to come to him at the end of every night, before dawn, and say: "As-salamu 'alayka ya Nabiyy Allah (Peace be upon you, O Prophet of Allah)." If he cleared his throat I would go back to my family, otherwise I would enter upon him.'"
Hafiz Muhammad Ameen
1214. Commentary: The editor of the book has declared the first two narrations as authentic (sahih) and the third as sound (hasan), but according to other scholars, this ruling is questionable because these narrations are, firstly, disconnected (munqati‘), and secondly, both in chain and text, they are inconsistent (mudtarib). Therefore, all three narrations are weak (da‘if). The basis of these narrations is ‘Abdullah ibn Najji, who is a narrator about whom there is criticism. Yahya ibn Ma‘in states: ‘Abdullah ibn Najji did not hear this narration from ‘Ali radi Allahu anhu. For details, see: [ذخیرة العقبیٰ شرح سنن النسائي : 14/225]
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 1214
Narrated Ali (RA): "I had the permission of Allah's Messenger (ﷺ) to see him in his house at two times, and whenever I entered to him while he was praying he would clear his throat as a sign to me." [Reported by an-Nasa'i and Ibn Majah].
Shaykh Safi ur-Rahman Mubarakpuri
Lexical Explanation: «مَدْخَلَانِ» "Meem" and "Kha" both have a fatha, and the "Dal" in between is sakin (silent). There are two times for presenting oneself in his service. «تَنَحْنُحِ» To circulate the sound in the throat; to clear the throat.
Benefit: From this hadith, it is understood that during prayer (salah), making such a sound out of necessity in which there is no articulation of letters does not invalidate the prayer.
Source: Bulugh al-Maram: Commentary by Safiur Rahman Mubarakpuri, Page: 176