Why he told them not to arrive unannounced
The reason given is on the other side of the door.
أَمْهِلُوا حَتَّى تَدْخُلُوا لَيْلاً ـ أَىْ عِشَاءً ـ لِكَىْ تَمْتَشِطَ الشَّعِثَةُ وَتَسْتَحِدَّ الْمُغِيبَةُ
Amhilū ḥattā tadkhulū laylan, li-kay tamtashiṭa-sh-shaʿithatu wa tastaḥidda-l-mughībah
Wait until you enter at night, so that the one with unkempt hair can comb it and the one whose husband has been away can prepare herself.
Where this comes from
An army coming home early sounds like a kindness. He stopped them at the edge of the city for the sake of people who had not been expecting them yet.
The full narration
Narrated Jabir bin `Abdullah:While we were returning from a Ghazwa (Holy Battle) with the Prophet, I started driving my camel fast, as it was a lazy camel A rider came behind me and pricked my camel with a spear he had with him, and then my camel started running as fast as the best camel you may see. Behold! The rider was the Prophet (ﷺ) himself. He said, 'What makes you in such a hurry?" I replied, I am newly married " He said, "Did you marry a virgin or a matron? I replied, "A matron." He said, "Why didn't you marry a young girl so that you may play with her and she with you?" When we were about to enter (Medina), the Prophet (ﷺ) said, "Wait so that you may enter (Medina) at night so that the lady of unkempt hair may comb her hair and the one whose husband has been absent may shave her pubic region
Source
Ṣaḥīḥ al-Bukhārī · 5079
- Agreed upon as authentic (al-Bukhārī / Muslim)