بسم الله الرحمن الرحيم
The Holy Qur'an states;
وَمَا أَرْسَلْنَا مِن رَّسُولٍ إِلَّا لِيُطَاعَ بِإِذْنِ اللَّهِ ۚ وَلَوْ أَنَّهُمْ إِذ ظَّلَمُوا أَنفُسَهُمْ جَاءُوكَ فَاسْتَغْفَرُوا اللَّهَ وَاسْتَغْفَرَ لَهُمُ الرَّسُولُ لَوَجَدُوا اللَّهَ تَوَّابًا رَّحِيمًا
Meaning : We sent not the Messenger, but to be obeyed, in accordance with the will of Allah. If they had only, when they were unjust to themselves, come to the Messenger and asked Allah's forgiveness, and the Messenger had (also) asked forgiveness for them, they would have found Allah indeed Oft-returning, Most Merciful.(Al-Qur'an, Surah an-Nisa, 4:64)

This is a direct Divine order to seek intercession of Prophet Muhammad (Peace be upon him) and this verse is Mutlaq (not restricted to any time)

HADITH#1;
It is narrated from Anas RA, "If there was a drought, 'Umar ibn al-Khattab would ask al-'Abbas ibn 'Abdu'l-Muttalib to do the rain prayer. He would say, 'O Allah, we seek intercession with You by Your Prophet SAW and we ask You for rain. We seek intercession with You by the uncle of our Prophet, so give us rain!'" He added, "And they were given rain."
Ref: Sahih Bukhari Book:Al istasqa Chapter: Suaal An Nas Al Imam Alistasqa iza Qahatu Page : 245 Hadith number :1010)

HADITH#2;
It was narrated from 'Uthman bin Hunaif that a blind man came to the Prophet (Peace be upon him) and said: "Pray to Allah to heal me." He said: "If you wish to store your reward for the Hereafter, that is better, or if you wish, I will supplicate for you." He said: "Pls Supplicate." So He(SAW) told him to perform ablution and do it well, to pray two Rak'ah, and to say this supplication: "Allahumma lnni as'aluka wa atawajjahu ilaika bimuhammadin nabiyyir-rahma. Ya Muhammadu inni qad tawajjahtu bika ila rabbi fi hajati hadhihi lituqda. Allahumma fashaffi’hu fiya (O Allah, I ask of You and I turn my face towards You by virtue of the intercession of Muhammad the Prophet of mercy. O Muhammad, I have turned to my Lord by virtue of your intercession concerning this need of mine so that it may be met. O Allah, accept his intercession concerning me)"
Ref : Ibn Majah transmitted it in his Sunan, book of Iqamat al-salat wa al-sunnat (establishing prayer and its sunnahs)[Page 197, Hadith No#1385) & Tirmidhī in al-Jami-us-sahīh, book of da‘awat (supplications) ch.119 (5:569#3578)
There are more Hadiths narrated on Tawassul.
May Allah SWT grant us Shafa'at (Intercession) of our beloved Prophet SAW on the day of Qiyamah..ameen!