Although Hazrat Umar (radhiyallahu ‘anhu) extended the musjid, he did not bring about any change in the actual facade and appearance of the structure. As Rasulullah (sallallahu ‘alaihi wasallam) had done, Hazrat Umar (radhiyallahu ‘anhu) also built the musjid with unbaked bricks. He placed the trunks of date palms as its pillars and erected the roof from the branches and leaves of date palms. He upheld the former simplicity of the original structure.
During the period of his khilaafah, Hazrat Uthmaan (radhiyallahu ‘anhu) also extended the musjid, and instead of unbaked bricks, he reconstructed the musjid using engraved stones and lime as mortar. He made the pillars from stone and built the roof using a kind of hardwood timber.
When Hazrat Uthmaan (radhiyallahu ‘anhu) decided to reconstruct the musjid in this manner, the Sahaabah (radhiyallahu ‘anhu) found it difficult to see the simplicity and original appearance of Musjid-e-Nabawi undergo these changes. When Hazrat Uthmaan (radhiyallahu ‘anhu) noticed the Sahaabah’s (radhiyallahu ‘anhum) continued disapproval and unhappiness over this, he addressed them in one of his sermons saying:
إنكم أكثرتم وإني سمعت النبي صلى الله عليه وسلم يقول: من بنى مسجدا يبتغي به وجه الله بنى الله له مثله في الجنة
You people have expressed much disapproval (over the new construction of the musjid which I intend to do). (The reason for me intending to do what I have explained to you is that) I heard Rasulullah (sallallahu ‘alaihi wasallam) saying, “The one who builds a musjid for the pleasure of Allah, Allah will surely build for him a palace similar to it in Jannah.”
The construction commenced in the month of Rabee’ul-Awwal 29 A.H. and reached completion in the month of Muharram 30 A.H. Accordingly, the construction took at least ten months to reach completion.
Imaam Maalik (rahimahullah) reports:
When Hazrat Uthmaan (radhiyallahu ‘anhu) commenced the renovation of the musjid, Ka’b Ahbaar would make dua saying, “O Allah! Do not allow this construction to reach completion!” When people enquired the reason for him making dua like this, he replied, “The moment this renovation reaches completion, the fitnahs (divine tribulations) will start to descend from the sky.”
(Extracted from Seeratul Mustafa 1/432-433)