If you are using dhcpcd (the client daemon, most people here are confusing it with DHCP and DHCPd which is different), then add the following text on the bottom of /etc/dhcpcd.conf

static interface eth0 static ip_address=192.168.1.135/24 static routers=192.168.1.1 static domain_name_servers=8.8.8.8

Of course remember to replace the IP info with your network details.

If you are using dhcpcd (the client daemon, most people here are confusing it with DHCP and DHCPd which is different), then add the following text on the bottom of /etc/dhcpcd.conf

static
interface eth0
static ip_address=192.168.1.135/24
static routers=192.168.1.1
static domain_name_servers=8.8.8.8

Of course remember to replace the IP info with your network details.