Kural - 718

To speak where understanding hearers you obtain,
Is sprinkling water on the fields of growing grain

Explanation
Lecturing to those who have the ability to understand (for themselves) is like watering a bed of plants that are growing (of themselves).

Tamil Transliteration
Unarva Thutaiyaarmun Sollal Valarvadhan
Paaththiyul Neersorin Thatru.