It is better to bathe before assuming Ihram
Question :
If the person who wants to perform Hajj comes down from Makkah to Mina on the 8th of Thul-Hijjah and he bathes at Mina, does that suffice him, and what is due upon him?
Answer:
If he bathed at Mina, there is no harm on him concerning that. However, it is better for him to bathe before assuming his Ihram in his house or in any place in Makkah. Then he should assume Ihram for Hajj at his place (in Makkah). He does not need to enter the Sacred Mosque (Al-Masjid Al- Haram) for Tawaf, because the person who goes out to Mina on the day of Tarwiyyah does not have to perform a Farewell Tawaf (Tawaful-Wada'). If he assumes Ihram without bathing, there is no problem with that. If he bathes after that in Mina while he is in the state of Ihram, there is no harm in that. However, it is better and it is the Sunnah that his bathing be before he enters Ihram. If he does not bathe, but instead assumes Ihram without bathing or without Wudhu' only, there is no problem with that, because the bathing is a Sunnah and the Wudhu' is a Sunnah in this case.
Source:
Ash-Shaykh Ibn Baz
Fatawa Islamiyah, Vol. 4 Page 112