Early Muslim armies stayed in encampments away from cities because Umar feared that they might get attracted to wealth and luxury. In the process, they might turn away from the worship of God and start accumulating wealth and establishing dynasties. When Uthman ibn al-Affan became very old, Marwan I, a relative of Muawiyah I, slipped into the vacuum, became his secretary, slowly assumed more control and relaxed some of these restrictions. Marwan I had previously been excluded from positions of responsibility. In 656, Muhammad ibn Abi Bakr, the son of Abu Bakr, the adopted son of Ali ibn Abi Talib, and the great grandfather of Ja'far al-Sadiq, showed some Egyptians the house of Uthman ibn al-Affan. Later the Egyptians ended up killing Uthman ibn al-Affan.
Who assumed power when Marwan I became very old?