Dear brother,
After greetings,
The narration in the book Al-Dawla (The State) and in Ahkam al-Salah (The Rules of Prayer) are not regarding a single incident:
- In the book Al-Dawla:
كتب مصعب إلى رسول الله صلى الله عليه وسلم يستأذنه أن يجمِّع بهم فأذن إليه ... فجمَّع بهم مصعب بن عمير في دار سعد بن خثيمة وهم اثنا عشر رجلاً
"Mus'ab wrote to the Messenger of Allah (saw) asking his permission to lead them in Friday prayer (yujammi'a bihim), and he gave him permission... so Mus'ab ibn Umayr led them in Friday prayer in the house of Sa'd ibn Khaythama and they were twelve men."
- In the book Ahkam al-Salah: Mus'ab led them in Friday prayer in the house of As'ad ibn Zurara, and in the Hadith:
كم كنتم يومئذٍ قال أربعون رجلاً
"How many were you on that day? He said: Forty men."
The first was in the house of Sa'd ibn Khaythama.
The second was in the house of As'ad ibn Zurara.
It appears that the Friday prayer was held in the two houses on two separate Fridays.
Accept my greetings.
05/01/2004 CE