What is the interpretation of the Hadith: "I was the guest of the Prophet, may Allah bless him and his family, and my mustache had grown long, so he trimmed it for me with a Siwak"? And how did Al-Mughirah ibn Shu'bah's mustache become after the trimming, and does this indicate the obligation of trimming until only the outline is visible?

The Hadith mentioned by the questioner was narrated by Abu Dawud and others, and authenticated by Al-Albani. Its wording states that the Prophet, peace and blessings be upon him, ordered a grilled side (of meat), and he took a knife to cut a piece for Al-Mughirah from it. Then Bilal came and notified him of the prayer. So he (the Prophet) cast aside the knife and said, "What is wrong with him? May his hands be soiled (with dust)!" And he stood up to pray. Al-Anbari added that the Prophet trimmed Al-Mughirah's mustache using a miswak, by placing the miswak under the mustache and cutting what extended and grew long from the hair. This is what many scholars have adopted, which is to trim the mustache so that the outline of the upper lip becomes visible. This is supported by his, peace and blessings be upon him, saying: "And trim the mustache."