It is commonly heard that if one performs Umrah once, then performing Hajj becomes mandatory (Wajib) for him. Please clarify this point.
This is not correct. Merely performing Umrah does not make Hajj mandatory. But if a person who did not perform Hajj before he reaches Makkah for any reason in the month of Shawwal or anytime thereafter before the 10th of Zilhijjah and he has resources to stay there upto the days of Hajj, only in that case it becomes obligatory on him to perform Hajj either that very year or in any subsequent year.
Fatawa by Mufti Muhammad Taqi Usmani.