Where should a women stand when performing Salah behind a male imam for example if it's just a husband & wife performing Salah? Should she stand on the side of him or directly behind him?
When a husband and wife pray together at home,
the woman must stand behind the man, not beside him.
This is the clear Sunnah of the Prophet ﷺ and the practice of the Sahabah.
A single woman counts as
one complete row by herself.
Evidences
A single woman forms a row by herself
Hadith of Anas رضي الله عنه
He said:
Arabic:
صليت أنا ويتيم في بيتنا خلف النبي صلى الله عليه وسلم، وأمي أم سليم خلفنا
Translation:
"I and an orphan prayed behind the Prophet ﷺ in our house, and my mother Umm Sulaim stood behind us."
Reference: Sahih al Bukhari: 727, Sahih Muslim: 658
Reference: Khulasat al Ahkam: 2/680
The man stands beside the imam, and the woman stands behind
Hadith of Anas رضي الله عنه
He said:
Arabic:
إن رسول الله صلى الله عليه وسلم صلى به وبأمه، أو خالته، قال فأقامني عن يمينه، وأقام المرأة خلفنا
Translation:
"The Messenger of Allah ﷺ led me and my mother or aunt in prayer. He placed me on his right and he placed the woman behind us."
Reference: Sahih Muslim: 660
Another narration confirming the same ruling
Anas رضي الله عنه said:
Arabic:
صلى بي رسول الله صلى الله عليه وسلم وبامرأة من أهلي، فأقامني عن يمينه، والمرأة خلفنا
Translation:
"The Messenger of Allah ﷺ led me and a woman from my family in prayer. He placed me on his right and the woman behind us."
Reference: Musnad Ahmad: 1/302
Reference: Sunan an Nasai: 805
Authenticated by Ibn Khuzaymah (1537) and Ibn Hibban (2204).
Additional Supporting Narrations
These narrations show that even when only
three people are present, with one woman,
she always stands behind.
Prayer at the death of Umair (the son of Abu Talhah)
Abdullah ibn Abi Talhah رحمه الله reports:
Arabic:
فتقدم رسول الله صلى الله عليه وسلم، وكان أبو طلحة وراءه، وأم سليم وراء أبي طلحة، ولم يكن معهم غيرهم
Translation:
"The Messenger of Allah ﷺ stood forward, Abu Talhah stood behind him, and Umm Sulaim stood behind Abu Talhah. No one else was with them."
Reference: Sharh Maani al Athar: 1/508
Reference: Mustadrak al Hakim: 1/365
Al Hakim said it is sahih on the conditions of Bukhari and Muslim, and adh Dhahabi agreed.
Instruction to a newly married man
Abu Saeed رحمه الله narrated:
Arabic:
وأمراني إذا أتيت بامرأتي أن أصلي ركعتين، وأن تصلي خلفي إن فعلت
Translation:
"They instructed me that when I go to my wife, I should pray two rakahs, and if she also prays, she should pray behind me."
Reference: Al Awsat by Ibn al Mundhir: 4/156
Reference: Musannaf Ibn Abi Shaybah: 2/217