There is an infinite number of teachings on this, among them this quotation from the Tantra of the Jewel That Embodies All:
Better than meditating for a hundred thousand kalpas
On one hundred thousand deities
Is remembering one’s teacher for a moment.
The merit of this is infinite.

The Tantra of the Array of Commitments says:
Better than meditating for one hundred thousand kalpas
On Buddhas with all the major and minor marks
Is thinking of one’s teacher for a single instant.
Better than a million recitations of the approach and accomplishment
Is a single prayer to the teacher.

In the Array of the Sublime we read:
Those who visualize their gracious teachers
On the crowns of their heads, in the centers of their hearts,
Or in the palms of their hands
Will hold even the accomplishment
Of a thousand Buddhas.

And in the Clarifying Lamp:
Fortunate disciple, far superior to the mass of merit that comes from the diamond body, speech, and mind of all the Bhagavan Buddhas — all those victorious, virtuous, transcendent Buddhas who dwell in the ten directions—is the merit of a single one of the master’s pores. Why is this? Fortunate disciple, bodhichitta is the essence of the Buddhas’ wisdom.

• Dudjom Rinpoche – A Torch Lighting the Way to Freedom – Shambhala Publications