Will the drying up of Lake Tiberias happen at the time of the Mahdi?
Question

I heard that drying of Sea Of Galilee is the sign of coming al-Mahdi. Is this true? is drying caused by drinking the sea water by Ya’jooj and Ma’jooj? How Sea Of Galilee is related to all these?
Praise be to Allah.The drying up of Lake Tiberias
The Sea of Galilee is also known as Lake Tiberias.
The hadiths indicate that the drying up of Lake Tiberias will be caused by Ya’jooj and Ma’jooj drinking its water. That will happen after the time of the Mahdi ends.
According to the marfoo‘ hadith narrated by Muslim in his Saheeh (2937) from an-Nawwaas ibn Sam‘aan (may Allah be pleased with him), the Prophet (blessings and peace of Allah be upon him) said: “… And Allah will send Ya’jooj and Ma’jooj, who will swarm down from every slope. The first of them will pass by the Lake of Tiberias and will drink what is in it, and the last of them will pass it and say: There was once water here. Then the Prophet of Allah ‘Eesa and his companions will be besieged until the head of the bull of one of them will be better than one hundred dinars for one of you today. The Prophet of Allah ‘Eesa and his companions will beseech [Allah], and Allah will send worms in their necks, and they will all perish as one in the morning. Then the Prophet of Allah ‘Eesa and his companions will come down to the earth and they will not find a spot the size of a handspan on earth that is not filled with their putrefaction and stench. The Prophet of Allah ‘Eesa (peace be upon him) and his companions will beseech Allah, and Allah will send birds like the necks of Bactrian camels which will carry them and throw them wherever Allah wills. Then Allah will send rain which will not be kept out by any house of clay or hair; it will wash the earth and leave it like a mirror…”
The emergence of Ya’jooj and Ma’jooj will occur at the time of ‘Eesa (peace be upon him), after ‘Eesa (peace be upon him) kills the Dajjaal. What appears to be the case is that this will only happen after the time of the Mahdi has ended, because there is no mention of the Mahdi in these events.
In the hadith of an-Nawwaas ibn Sam‘aan it says that the Prophet (blessings and peace of Allah be upon him) told them a hadith about the Dajjaal, in which he said: “Whilst they are like that, Allah will reveal to ‘Eesa (upon him): I have brought forth some slaves of Mine [Ya’jooj and Ma’jooj] against whom no one will be able to fight; take My [believing] slaves to safety in at-Toor. And Allah will send Ya’jooj and Ma’jooj, who will swarm down from every slope. The first of them will pass by the Lake of Tiberias and will drink what is in it, and the last of them will pass it and say: There was once water here.” Narrated by Muslim (2937).
Ibn Abi’l-‘Izz al-Hanafi (may Allah have mercy on him) said:
Ya’jooj and Ma’jooj will emerge during the time of the Messiah ‘Eesa ibn Maryam (peace be upon him), after he kills the Dajjaal, and Allah will destroy them all in a single night, by virtue of ‘Eesa’s supplication against them."  (Sharh at-Tahhaawiyah p. 514).
Conclusion:
The drying up of the Sea of Galilee, which is also known as Lake Tiberias, is not a sign of the emergence of the Mahdi; rather it will only happen after the time of the Mahdi has ended, and it will happen as a result of the actions of Ya’jooj and Ma’jooj.
And Allah knows best.